The man who brokered peace in Northern Ireland says a hard border must be avoided or violence is possible.

George Mitchell, a former US senator, told Sky News it would be complacent to dismiss the risks posed by Brexit as scaremongering.

"Reason, common sense and prudence tells us that in light of the history of Northern Ireland, no steps should be taken that might make a resumption of violence more possible. I think that's the best way you can frame it," Mr Mitchell said.

It is more than 20 years since Mr Mitchell, a former US peace envoy to Northern Ireland, uttered the historic words: "I am pleased to announce that the political parties of Northern Ireland have reached agreement."

Image:Bill Clinton and George Mitchell received The Freedom Of The City Of Belfast last year

The EU says the backstop protects the Good Friday Agreement. Brexiteers say it breaches it by risking Northern Ireland's place in the UK.

The backstop refers to a customs plan to avoid a "hard" border between Ireland and Northern Ireland if a Brexit deal is not reached.

"Governments should not make public promises lightly and when they do make them, they should keep them," he said.

"It was over a year ago that the government of the United Kingdom and… the European Union made a public pledge that whatever the outcome of their negotiations, there would not be a resumption of a hard border in Northern Ireland."